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Dust Grains Growing Around Stars in the Orion Nebula

January 10, 2006

WASHINGTON, D. C. — New observations of the Orion Nebula at infrared wavelengths reveal that small dust grains located in disks around young stars are growing, taking the initial steps toward forming planets despite bathing in a flood of radiation from highly luminous stars.

The properties of dust in disks around young stars plays a pivotal role in understanding star formation and determining the origins of planets in our Solar system and in extrasolar planetary systems as well. The results are presented today at the 207th meeting of the American Astronomical Society in Washington, D. C.

“One of the key questions we are trying to address is whether or not planets can form around young stars in the seemingly hostile environment of the Orion Nebula,” said Dr. Marc Kassis, support astronomer at the W. M. Keck Observatory and lead author of the poster sharing the results.

The Orion Nebula, located about 1500 light years away, is an energetic stellar nursery giving birth to thousands of young, Sun-like stars with protoplanetary disks. But a few of these newborn stars are 10 to 30 times the mass of our Sun and 10,000 times as bright. These massive stars bathe the entire region in harsh ultraviolet radiation which evaporates the protoplanetary disks of their lower mass neighbors.

“You would think that the strong ultraviolet radiation that is evaporating these disks would also inhibit planet formation, but the larger particles we see in these Orion disks seem to suggest otherwise,” said team member Dr. Nathan Smith, Hubble Fellow at the University of Colorado.

To determine the relative sizes of the grains in these protoplanetary disks, the research team used the Long Wavelength Spectrometer on the Keck I 10-meter telescope and the Mid-Infrared Spectrometer and Imager at the 3-meter NASA Infrared Telescope Facility, both situated 14,000 feet atop Mauna Kea on the island of Hawai`i.

In the optical part of the spectrum, these protoplanetary disks are dark and are sometimes viewed in silhouette against the bright nebula. In contrast, the dusty disks are extraordinarily bright in the infrared (see Fig 1). The observations revealed broad spectral signatures of silicate grains, and the overall shape of the spectra was unlike the silicate emission of relatively smaller grains typical of the interstellar medium.

“The silicate profiles from the protoplanetary disks are generally flat-topped instead of peaked, indicating the grains have increased in size since the birth of these disks,” said Dr. Kassis. “You wonder whether the grains will grow enough to start forming planets.”

“Could our own solar system have formed in such an environment?” posed Dr. Ralph Shuping, support scientist for the Stratospheric Observatory for IR Astronomy (SOFIA). “Careful study of primitive materials in meteorites suggests that it was, and our observations show that the initial stages of grain growth that lead to planet formation can occur in protoplanetary disks born in Orion-like environments.”

Most stars are born in clusters with bright, massive stars relatively nearby. The stars in clusters and their protoplanetary disks born in regions like the Orion Nebula can be exposed to the intense ultraviolet radiation from massive stars, stellar winds, jets, gravitational pulls from their neighbors, and supernova explosions. Yet, recent theoretical work and the study of primitive meteorites indicate that our Solar System may have been born in a region like the Orion Nebula.

“Some years ago, we thought ultraviolet radiation would be hazardous to disks,” said Dr. John Bally at the University of Colorado. However, recent work by Drs. Henry Throop of the Southwest Research Institute and Bally showed that ultraviolet irradiation could promote the rapid formation of planets. “So, in disks where grains have grown and settled to the disk mid-plane, ultraviolet radiation can remove gas, leaving large particles behind to accumulate through their mutual gravitation into small, planet-like objects,” added Dr. Bally.

The team’s observations also hint at the composition of the grains. From details in the shape of the infrared spectra, the team is identifying the presence of silicate minerals such as olivine and fosterite; olivine being the same mineral found along the green sand beaches in Hawai`i.

“It’s amazing to think that we can study the minerology of these tiny grains 1500 light years away!” remarked Dr. Shuping.

The team responsible for the discovery of grain growth in Orion Nebula protoplanetary disks is Ralph Shuping (USRA-SOFIA), Marc Kassis (W. M. Keck Observatory), Mark Morris (UCLA), and Nathan Smith and John Bally (University of Colorado). The team acquired data at NASA’s IRTF through a collaboration with the instrument team that includes Joseph Adams (Cornell University), Joseph Hora (Harvard-Smithsonian Center for Astrophysics), James Jackson (Boston University), and Eric Tollestrup (UH-IfA, NASA IRTF).

This work was supported by the Colorado Center for Astrobiology and the UCLA Center for Astrobiology, both supported by the NASA Astrobiology Institute. The Infrared Telescope Facility is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program. Some of the observations for this research were provided by the W. M. Keck Observatory using Director’s discretionary time, also known as “Team Keck.” The W. M. Keck Observatory is operated by the California Association for Research in Astronomy (CARA), a non-profit 501 (c) (3) corporation whose board of directors includes representatives from the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.

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Dust Grains Growing Around Stars in the Orion Nebula


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